[Exim] 550 postmaster error

I have exim running on a RH7.3 machine. Works great!

When I run a dns check at dnsreport.com for my domain, everything is great.
When I try to send an email to postmaster@IPADDRESS, I get an error:
<root@[IP]>: host IP[IP] said: 550-Host
    smtp.dc.net [205.166.61.173] is not permitted 550-to relay through
    domain.com.  550-Perhaps you have not logged into the pop/imap
    server in the last 30 minutes. 550-You may also have been rejected because
    your ip address 550-does not have a reverse DNS entry. 550 relaying to
    <root@[IP]> prohibited by administrator

postmaster@??? works fine though. What am I doing wrong?


Thanks,
Chuck